Minivib P-wave Seismic Reflection Experiments in the Pacific Northwest and New
Madrid Seismic Zone
The overall goal of these experiments is to provide high-resolution images of the
sediment structure and faults in the Arkansas portion of the New Madrid seismic zone
and the Puget Sound region, Washington. These studies are motivated by the need to: (1)
better understand the sedimentary basin structure and detect faults that may be hidden
under the recent alluvial cover in these seismically active areas, and (2) provide key
constraints on earthquake hazard assessments in these large, at risk areas, and lead to
improved earthquake ground motion simulations that earthquake engineers could use to
assess potential losses. This collaboration promotes the basic goal of NEES "to engage
and support earthquake risk-mitigation partners in experimental activities that improve
the seismic design and performance of our infrastructure systems through the integration
of people, ideas, and tools".
Experiment 1 consisted of one continuous approximately 14-km-long P-wave seismic
reflection profile along Steamboat Island Road, about 11 km west-northwest of Olympia,
Washington. The line was sited to cross the projection of the Olympia structure, a gravity
and magnetic anomaly possibly caused by an active fault. These seismic data provide the
first image of stratigraphic deformation of Tertiary bedrock from outcrop in the Black
Hills, Washington, northward toward Puget Sound. A draft of the interpretive results is
currently in preparation. An abstract submitted to the Seismological Society of America
2014 Annual Meeting is presented here:
High-Resolution Geophysical Imaging Across the Olympia
Structure, Southern Puget Lowland, WA: Defining a Fault
System
ODUM, J. K., U.S. Geological Survey, Golden, CO, odum@usgs.gov; STEPHENSON, W. J., U.S.
Geological Survey, Golden, CO, wstephens@usgs.gov; PRATT, T. L., U.S. Geological Survey,
Reston, VA, tpratt@usgs.gov; and BLAKELY, R. J., U.S. Geological Survey, Menlo park, CA,
blakely@usgs.gov
Linear magnetic anomalies suggest that the 80-km-long Olympia structure in the southern
Puget Sound is a fault vertically offsetting Eocene Crescent Formation as much as 6 km.
Northeast of the Olympia structure, Crescent Formation lies buried beneath 4-6 km of
Tertiary and Quaternary strata of the Tacoma basin, whereas Crescent Formation is
exposed at the surface immediately south of the Olympia structure. Although numerous
marine seismic-reflection profiles have been acquired near the projected trace of the
Olympia structure, its tectonic character remains enigmatic because shallow inlets of
southern Puget Sound prevent the collection of deep-penetration marine seismic profiles
across the geophysical anomalies. To supplement the existing marine data, we acquired
14.6 km of land-based seismic reflection data in a profile that extends from exposed
Crescent basement in the Black Hills northward across the surface projection of the
Olympia structure inferred from potential field anomalies. The reflection seismic data
image the Crescent bedrock surface to approximately 1 km depth beneath the Tacoma
basin and reveal the dip on this surface to be no greater than about 8 degrees. Although
regional potential field data show a strong linear trend for the Olympia structure that
implies a simple thrust fault, high resolution magnetic and gravity modeling along the
land-based profile suggests that the structure is complex and possibly segmented.
Assuming normal magnetization, contacts determined from boundary analysis of
magnetic anomalies agree in both location and sense of movement with faults identified
on the seismic profile. We propose that the seismic and potential-field data identify
oblique-slip faults along the northernmost part of the Black Hills uplift. Our data further
suggests that deformation of the Olympia structure changed from largely transpressive to
dextral strike-slip in recent geologic time.
Information about data:
Data are archived in the SEGY format, a standard of the Society of Exploration
Geophysicists. Numerous proprietary and open-source programs that read the SEGY
format are readily available (e.g., Seismic Unix, from the Center for Wave Phenomenon
at the Colorado School of Mines). Coordinates for seismic stations are provided in the
SEGY headers for each trace, and are provided in UTM Zone 10N projection. The data
are provided in uncorrelated format; the pilot trace is the vibroseis signal using a 15-to160 Hz 12-second sweep length and two second “listen time”. Sample rate for the data is
2 milliseconds. Source and receiver spacings were nominally 5 meters, and sensors were
8-Hz natural frequency vertical-component geophones. The P-wave seismic profile was
acquired from ~1 km south of Steamboat Island southward along Steamboat Island Road,
crossing U.S. Highway 101, and ending along exposed outcrops of basaltic bedrock in the
Black Hills (Figure 1). Observer’s logs (field recording notes of acquisition equipment
operator) are provided in pdf format, and these notes give detail information for source
and receiver station locations.
Figure 1. Location of seismic profile SI1 (shown by red line). Waterways shown are
southern extent of Puget Sound, Washington.